Cushman et al. in U.S. Pat. No. 4,052,511 disclose that various carboxyalkanoyl prolines, pipecolic acids, and azetidine-2-carboxylic acids possess angiotensin converting enzyme inhibition activity.
Ondetti et al. in U.S. Pat. No. 4,105,789 disclose that various carboxyalkanoyl amino acids possess angiotensin converting enzyme inhibition activity.
Ondetti et al. in an article entitled "The Use Of An Active Site Model In The Design Of Specific Inhibitors Of Angiotensin Converting Enzyme", Peptides-Proceedings Of The Fifth American Peptide Symposium (1977), discuss the in vitro angiotensin converting enzyme inhibition activity of various compounds including 1-(L-4-amino-4-carboxy-1-oxobutyl)-L-proline.